Dolomite refractory products have been widely used for lining kilns such as LD converters, basic furnaces for steelmaking, cement rotary kilns or the like. Currently there is an increasing demand for development of refractory products improved in spalling resistance, impact resistance and the like in view of recent tendency for narrowly limited kiln conditions including, for example in case of LD converters, use of larger size converters, rise of temperature of molten steel to be withdrawn from a converter, reduction of refining time, etc.
To cater for this new situation, for example basic refractory products predominantly containing magnesia (hereinafter referred to as MgO) have been developed. The proposed refractory products are excellent in corrosion resistance to high-basicity slag but have the serious drawback of being poor in spalling resistance. Owing to low reactivity between MgO and slag, slag is caused to penetrate into a refractory brick until it reaches the brick region having a temperature corresponding to the melting point of slag. Since the penetrated layer of slag weakens the texture structure of MgO crystals (periclase) present in this brick region, the reduction of brick temperature induces physical differences (in thermal expansion coefficient, apparent porosity, strength and so on) between the penetrated layer and original layer, causing likelihood for spalling to occur. Because MgO has a high thermal expansion coefficient, spalling tends to take place also due to the difference of thermal expansion between the side of bricks facing the kiln interior (high-temperature side) and the rear side thereof (low-temperature side). In a kiln lined with MgO bricks, thermal spalling and structural spalling arise from a combination of various factors such as action of slag, thermal deformation due to kiln structure, thermal shock and mechanical impact afforded by operational cycles of kiln (cooling and heating cycle), etc. with the result that the lining bricks are gradually exfoliated, eventually making the kiln inserviceable.
To overcome said drawbacks of MgO bricks, it has been proposed to use zirconia (preferably stabilized zirconia)-containing MgO-calcium oxide (CaO) refractory bricks (see Japanese Unexamined Patent Publication No. 96005/1974). The proposed bricks have reportedly improved structural spalling resistance which is imparted due to the phenomenon that the penetrated slag reacts with zirconia near the brick surface to become highly viscous, thereby preventing further slag penetration. However, the fact is that the proposed refractory bricks achieve substantially no improvement in thermal spalling resistance.
MgO-CaO refractory products containing MgO, dolomite and CaO have been developed and frequently used. Although with high strength, these refractory products remain unsatisfactory in spalling resistance, thermal shock resistance, etc. Especially when this refractory product is used for lining a cement rotary kiln, the following disadvantage is entailed. In this case, a coating composed of cement is applied on the surface of refractory product in the burning zone of kiln in an attempt to extend the service life of refractory product. However, the coating is caused to intermittently form and fall off in a vigorous manner by rotation of the kiln or the like, whereby the surface of refractory product is subjected to strong thermal shock, causing spalling.